There are a number of circumstances when an address to one volume of DASD memory requires a translation to a second address of another volume of memory on a separate DASD. Disaster protection systems implement a remote dual copy procedure wherein each reference to a primary DASD volume is duplicated by a relatively concurrent reference to a secondary DASD volume that is remote from the primary DASD. Remote in this context means at whatever distance will offer immunity from a disaster at the primary DASD location. Because system users require that the process of duplexing data not impact applications performance, an asynchronous copy procedure is implemented--as contrasted to a synchronous copy procedure. A synchronous copy procedure is one in which data is sent to a secondary DASD location and its entry therein is confirmed prior to ending the input/output operation at the primary DASD. An asynchronous procedure occurs when the input/output primary operation at the primary DASD is allowed to complete before data is confirmed to have been stored at the secondary DASD location. In performing such a copy procedure, address translation must be performed so that when an address is received for a primary DASD memory volume, an address for a secondary DASD memory volume can be found which corresponds to the primary DASD memory volume. Classically, translation tables have been used to perform such address references, however they have occupied large areas of memory due to the large volumes of stored data.
Requirements for continuous real-time data duplexing also occur when sending updated data from a primary DASD to a secondary DASD as a continuous stream. When such data is identified by its primary DASD address (e.g. device, cylinder and track), the secondary DASD must perform a continuous translation of the primary DASD addresses to secondary DASD addresses. Continuous translation of DASD addresses is also a requirement in systems that implement virtual volumes where the actual memory address at which data is stored is different from the address which requesting software uses to identify a particular track. While the art is replete with schemes for effecting different types of address translation, each procedure requires a tradeoff between the amount of translation table memory and the speed of lookup in the translation tables.
Accordingly, it is an object of this invention to provide an improved method and system for enabling translation of a set of DASD addresses in a first DASD volume to a set of addresses in a second DASD volume.
It is another object of this invention to provide an improved DASD address translation procedure which is highly efficient in use of memory space.
It is yet another object of this invention to provide a DASD address translation method and system which exhibits a high speed translation capability.